There are devices for fluid power recuperation in the form of hydropneumatic accumulators (hereinafter—accumulators), their shells containing a variable volume gas reservoir filled with pressurized gas via a gas port as well as a variable volume fluid reservoir filled with fluid via a fluid port; while these gas and fluid reservoirs are separated by a separator movable relative to the shell.
Fluid power recuperation is performed using accumulators both with a solid separator in the form of a piston and elastic separators, for example, in the form of elastic polymer membranes or bladders as well as in the form of metal bellows.
Before operation the gas reservoir of the accumulator is charged via the gas port with pressurized gas, generally, with nitrogen, up to the initial pressure from several to dozens MPa.
During power transfer from the fluid power system to the accumulator (at in hydraulic hybrid vehicle braking, for example) the pumping of the working fluid from the fluid power system into the accumulator occurs as well as working gas compression within the accumulator, with a gas pressure increase and a temperature increase. During power return from the accumulator into the fluid power system (at acceleration of the hydraulic hybrid vehicle, for example) the pressurized working gas expands and displaces the working fluid into the fluid power system.
As a rule, an accumulator contains one gas reservoir and one fluid reservoir with equal gas and fluid pressures in them. The higher the hydraulic power transferred to the accumulator, the higher the gas compression ratio in it. To maintain the required recuperated power the pressure increase has to be compensated by reduced delivery of the hydraulic machine (a pump or a motor) hydraulically connected with the accumulator. As the delivery is reduced, the hydraulic machine efficiency drops; hence, the total recuperation efficiency drops, which is a disadvantage of such devices.
An increased volume of the accumulator or an increased number of accumulators for gas compression ratio reduction raises the cost of the system, as well as makes it heavier. Weight is critical for mobile applications.
A well-known device is used to reduce gas compression ratio and, at the same time, to increase the maximum possible recuperated power. The device includes a hydropneumatic accumulator, its shell containing a fluid port communicating with the fluid reservoir of the accumulator separated by a movable separator from the gas reservoir of the accumulator that communicates with at least one gas receiver via a gas port.
When the working fluid is pumped from the fluid power system into the fluid reservoir of the accumulator, the separator is displaced and forces the gas out of the accumulator into the receiver, compressing the gas in the receiver and in the accumulator. The work of pumping the fluid into the accumulator is transformed into internal energy of the pressurized gas, its pressure and temperature increasing. When the power returns from the device into the fluid power system, the pressurized working gas expands and is partially forced out of the receiver into the gas reservoir of the accumulator. The separator is displaced, the volume of the fluid reservoir of the accumulator decreases and the working fluid is forced out of the fluid reservoir into the fluid power system via the fluid port. The internal energy of the pressurized gas is transformed into the work of the fluid displacement, i.e. the device returns the fluid power received from the fluid power system back into the system, with the gas pressure and temperature decreasing.
The addition of the receiver that is lighter and cheaper than the accumulator to the system allows increasing the amount of the recuperated power at the expense of better use of the accumulator volume and reducing gas compression ratio and, accordingly, the range of variation of the delivery of the hydraulic machines operating in the system, which increases the recuperation efficiency.
A shortcoming of the devices used for fluid power recuperation is the high level of heat losses due to the fact that when compressed and expanded the gas in the receiver exchanges its heat only with the internal walls of the receiver, the distance between such walls for typical receiver volumes (units and dozens of liters) being too large (dozens and hundreds mm) and the gas heat conductivity being too small.
At such distances the gas heat exchange with the receiver walls caused by the gas heat conductivity is insignificant. Therefore, the gas compression and expansion processes are essentially non-isothermal and there emerge considerable temperature gradients of dozens and even hundreds degrees in the receiver. Considerable temperature differentials in a large receiver volume generate convective flows increasing the heat transfer to its walls dozens and hundreds times. Therefore, the gas heated during compression in the receiver and partially in the accumulator, cools down, which results in reduced pressure of the gas and increased accumulated power losses during storage of the accumulated power (for example, when the hydraulic hybrid vehicle stops). The non-equilibrium heat transfer processes in case of high temperature differentials are irreversible, i.e. the greater part of the heat transferred from the pressurized gas to the receiver walls cannot be returned to the gas during expansion. Thus, when the gas expands, the amount of the fluid power returning to the fluid power system is much less than the amount received during gas compression.
Therefore, the above described device has low efficiency of the recuperated fluid power due to high heat losses.
The device of fluid power recuperation which may be the closest analog to the present innovation includes at least one hydropneumatic accumulator communicating with at least one gas receiver (gas bottle) via its gas port. The gas receiver is made in the form of aggregate of cells communicating with the gas port of the accumulator. For higher safety the cells have strong walls with considerable thermal capacity exceeding the thermal capacity of the gas in the receiver. The receiver cells are made in the form of narrow channels (tubes) so that the ratio between the receiver volume and the area of the internal surfaces of the cells does not exceed 10 mm, which both reduces the kinetic energy of the gas jet in case of cell shell destruction and increases the heat exchange between the gas and walls of the cells considerably. Thus, the walls of the cells in said receiver perform the function of a regenerating heat exchanger taking heat from the gas during compression and returning the heat to the gas during expansion, thus ensuring lower heat losses of the devices. The closer the walls of the cells to one another, i.e. the smaller the cross dimensions of the cells, the more efficient the heat exchange between them and the gas and the higher the recuperation efficiency. However, a shortcoming of the proposed device is the difficulty of manufacturing of the receiver in the form of a strong honeycomb structure, especially with small size cells necessary for higher heat exchange capacity with the growing recuperation rate. In addition, the proposed honeycomb structures are poorly compatible with commercial gas receivers of the most common type, i.e. made with solid cylindrical shells with rounded ends, the holes in them being much smaller than the diameters of internal chambers of the receivers.